relax allocated-memory tests for libcxx

Summary:
The current version of libc++ deallocates the existing capacity
when calling operator=(initializer_list), even if the map is empty. This
diff relaxes the expectations of F14's allocated memory tests (which
fall back to the underlying STL std::unordered_map on some platforms)
to accept this implementation.

Reviewed By: shixiao

Differential Revision: D17527441

fbshipit-source-id: 01da55aaeb6f8328e093251b026052dac365f7b2

folly/container/test/F14MapTest.cpp

@@ -107,7 +107,9 @@ void runAllocatedMemorySizeTest() {
     m.reserve(5);
     EXPECT_GT(testAllocatedMemorySize(), 0);
     m = {};
-    EXPECT_GT(testAllocatedMemorySize(), 0);
+    if (preciseAllocInfo) {
+      EXPECT_GT(testAllocatedMemorySize(), 0);
+    }
   }
   EXPECT_EQ(testAllocatedMemorySize(), 0);
   EXPECT_EQ(testAllocatedBlockCount(), 0);